This invention relates generally to metal-insulator-silicon field effect transistors (MISFETS), and more particularly the invention relates to a MISFET or MOSFET having a gate-drain shield positioned adjacent to and coplanar with the gate electrode.
The lateral double diffused metal-oxide-silicon field effect transistor (LDMOS) is used in power amplifiers for wireless applications such as in cellular telephones. The gate-to-drain feedback capacitance (Cgd or Crss) of any MOSFET device must be minimized in order to maximize RF gain and minimize signal distortion. C.sub.gd is critical since it is effectively multiplied by the voltage gain of the device as follows: EQU C.sub.effectve =C.sub.rss (1+gm R.sub.L)
where gm is the transconductance and R.sub.L is the load impedance of the device.
Heretofore the use of a source field plate to improve breakdown characteristics have been proposed for use with a LDMOS transistor. See for example, Okabe et al U.S. Pat. No. 4,172,260 and Adler et al U.S. Pat. No. 5,252,848. In each of these structures the source electrode is extended over the gate electrode and insulated therefrom to provide a Faraday shield. However, such a shield over the gate electrode does not optimize the gate-drain capacitance, C.sub.gd, since the shield does not effectively screen out the C.sub.gd component under the gate. Further, both structures require complex processing and are not suitable for use with vertical DMOS devices.
Weitzel U.S. Pat. No. 5,119,149 discloses a two-layered gallium arsenide structure and process in which a shield electrode is positioned adjacent to the gate but is not at the same level as the gate. Thus the gate-drain capacitance under the gate electrode is not optimally reduced. Also, the shield is not self-aligned to the gate and can cause variable shielding due to misalignment.
The present invention is directed to providing a lateral shield in an MOSFET power device including an extended drain MOSFET, a lateral DMOS transistor, and a vertical DMOS transistor using processes which are readily implementable.